A substantial number of factors affect the stability and calibration of electrophotographic imaging equipment such as printers and copiers. In general, a number of voltages are controlled to produce the required image density and other required properties. Such voltages include a voltage for charging a photoreceptor on which a latent image is formed, such as a roller voltage, a corotron voltage or a scorotron voltage. The voltage of the developer, both for liquid and powder toner development, is also controlled. Furthermore, control of the intensity of light used for selective discharge of the photoreceptor in forming the latent image is also important in optimal formation of the latent image. In laser printers, the intensity of light is controlled through control of laser power.
Repetitive use of the imaging apparatus requires systematic, gradual, changes in some of the factors mentioned above, such as the charge and discharge voltages of the photoreceptor to preserve proper operation of the system, while other factors are not dependent on time or the environment of the imaging apparatus.
Direct control of the physical parameters of the imaging apparatus has proven to be inadequate. Therefore, calibration methods are generally used for controlling the color of the printed image.
As known in the art, the color density of the final image generally depends on two factors, namely the optical density (OD) of solid printing and a look up table (LUT) of the imaging apparatus. The LUT is adapted to compensate mainly for the dot gain of the imaging apparatus, i.e. the difference between the actual, printed, dot area and the dot area defined by the corresponding digital input.
According to one known calibration technique, one of the voltages mentioned above is varied manually in accordance with variations in the solid optical density (OD) of the final image. For example, the voltage between the photoreceptor and the developer roller, also referred to as the "brightness voltage", may be varied in accordance with the solid OD of the final image. However, since the brightness voltage has an effect on the gray level density balance of the final image, this technique is insufficiently accurate for high quality printing.
Density-balance inaccuracies are particularly crucial in color printing, where the balance between colors is extremely sensitive to density balances within the different base colors, e.g. cyan, magenta, yellow and black. Therefore, complex calibration procedures, in which comprehensive adjustments are performed, must be frequently carried out on the imaging apparatus. Existing calibration procedures, which generally include the derivation of a new LUT, are highly time consuming, typically taking a few hours to perform.
Examples of existing calibration techniques are described in U.S. Pat. Nos. 4,839,722, 5,070,413, 5,258,810 and 5,262,825.